1. Field of the Invention
This invention relates to semiconductor read only memory devices and more particularly to method of manufacture of a Mask ROM device and the devices produced thereby.
2. Description of Related Art
In a conventional MASK ROM device, there are only two states, which are the "ON" state and the "OFF" state. The "ON" state is defined by code implantation. Typically the "OFF" cell threshold voltage (V.sub.T) is above 5 V and the "ON" cell V.sub.T is below 1 Volt. In a high density MASK ROM (32M or 64M), a conventional two state MASK ROM (which has a large chip size) will consume a large wafer area because of that large chip size.